Fang, Li, Liu and Zhang signed a loan contract, saying that: “Fang borrows 1 million yuan from Li, Liu mortgages his house to Li, and Zhang guarantees for this loan.” Everyone signed except Li. Liu gave the property ownership certificate to Li, promising to complete the mortgage registration a few days later. After Li handed over the 1 million to Fang, Fang did not pay it back on due date. 
Which of the following is correct? 
A、The loan contract is not formed.
B、Fang should return it as unjust enrichment.
C、Zhang should assume the responsibility as a guarantor.
D、Liu has no obligation to complete the mortgage registration.